This application relates to filtering and control of wavelength or frequency of optical signals, and more specifically, to techniques and devices for separating and combining optical signals of different frequencies or wavelengths.
Certain applications need to add or remove an optical spectral component at a selected wavelength in an optical signal. Various optical filters such as band-pass filters and other types of filters may be used to perform such operations.
One area of applications of adding or removing optical spectral components is wavelength-division multiplexing (WDM) devices and systems that use optical carriers of different wavelengths to transmit information in a single fiber. The WDM wavelengths are generally evenly spaced from one another according to a selected standard, such as the WDM wavelength grid by International Telecommunication Union. Such wavelength division multiple access technology can increase the information transmission capability of optical communication systems. In a WDM system, a channel of data may be added to other channels of data in a fiber link by adding an optical carrier at a selected wavelength that is modulated with the desired data. Conversely, a channel of data carried by an optical carrier may be xe2x80x9cdroppedxe2x80x9d off from other channels in the same fiber at a desired location in the fiber network for data extraction, signal switching and routing, signal conditioning, or other signal processing functions. An optical multiplexer is a device that can be used to add one or more optical channels to other channels in a single fiber link. An optical demultiplexer is a device that can be used to remove one or more optical channels from other multiplexed channels in a single fiber link.
The capacity of each WDM fiber link is in part determined by the number of wavelengths that can be multiplexed together. In general, the spectral spacing between two adjacent wavelength channels should be as small as possible so that the number of channels within a given spectral range can be as large as possible without causing adverse signal interference between two adjacent channels. Some WDM systems have a minimum channel spacing of about 50 GHz and are primarily limited by the wavelength resolution of their wavelength division multiplexers and demultiplexers. Such WDM multiplexers and demultiplexers may be built based on a number of filtering mechanisms, including dielectric coated narrow band filters, diffraction gratings, fiber gratings, integrated waveguide array, interleaved Mach-Zehnder interferometers, interleaved birefringent interferometers and others.
Channels with a spacing of 10 GHz or less are desirable for adding more wavelength channels in WDM systems to increase the overall transmission speed and capacity of the systems. In addition, narrower channel spacing can lower the cost of the optical transmitter and receiver modules because high-speed time division multiplexing in each transmitter and receiver can be greatly reduced. For example, for a channel spacing of 2.5 GHz, the corresponding transceiver""s speed can be reduced to 1.5 (=0.7xc3x972.5) GHz.
This application includes methods and devices for combining multiple optical signals with different optical wavelengths into a single fiber and for separating signals with different wavelengths in a single fiber into different signals in different optical paths, both based on their polarization properties. The channel spacing from a few megahertz to thousands of gigahertz may be achieved by such polarization-based multiplexing and demultiplexing. Such devices may be used in, among others, ultra-dense WDM systems, optical frequency division multiplexing systems, subcarrier carrier multiplexed (SCM) communication systems. Furthermore, the devices may be used to produce a strong pump beam by combining powers from many lasers with slightly different frequencies.